This invention relates to battery chargers, and more particularly, relates to techniques for charging lead-acid automotive-type batteries.
The most frequently used type of charger today is one in which an initially high current from a constant voltage source is allowed to decrease or taper downward as the battery's state of charge rises. This is the familiar "taper" charger. The battery is presumed by the operator to be properly charged when the downward tapering current attains some low level. But this low current level is greatly dependent upon battery size, condition, line voltage, as well as many other factors. In addition, many chargers have a multiple position switch and a timer which permit the operator to select the initial charging current level and charging time. If the initial current selected by the operator is high, then the final tapered current level will also be high. Under these conditions the operator may misjudge when a "proper" charge has been attained. Also, if the current selected by the operator is high and the charging time is made long, the battery may suffer irreversible damage due to over-heating and water loss and may even explode.
All lead-acid batteries consume water while being charged. The water is decomposed into its constituent gasses, hydrogen and oxygen, and is allowed to escape from the battery. This is normal and is not serious with the conventional or regular type of lead-acid battery because the water can easily be replaced. The newer "maintenance-free" batteries have a different construction and chemistry, and the amount of water lost per charge cycle is less. However, the cells of these batteries are sealed and are not accessible to the user. Thus, the water lost, even though it is less, cannot be replaced without destroying the seals. Also, the specific gravity of the electrolyte cannot be observed without destroying the seals (specific gravity is sometimes used as an indication of state of charge) and this indication of charge is unavailable from sealed, maintenance-free batteries. The sealed batteries are, therefore, more susceptible to errors of judgment then the conventional ones.
Taper chargers require frequent operator attention as well as judgment. If the attention is not always provided and the judgment is in error, the battery may be either insufficiently charged or damaged by overcharging.